The following article links have a summary available that was not added to the page:

Adaptation-Executers, not Fitness-Maximizers - A central principle of evolutionary biology in general, and evolutionary psychology in particular. If we regarded human taste buds as trying to maximize fitness, we might expect that, say, humans fed a diet too high in calories and too low in micronutrients, would begin to find lettuce delicious, and cheeseburgers distasteful. But it is better to regard taste buds as an executing adaptation - they are adapted to an ancestral environment in which calories, not micronutrients, were the limiting factor.

Your Strength as a Rationalist - A hypothesis that forbids nothing, permits everything, and thereby fails to constrain anticipation. Your strength as a rationalist is your ability to be more confused by fiction than by reality. If you are equally good at explaining any outcome, you have zero knowledge.

Policy Debates Should Not Appear One-Sided - Debates over outcomes with multiple effects will have arguments both for and against, so you must integrate the evidence, not expect the issue to be completely one-sided.

Availability - Availability bias is a tendency to estimate the probability of an event based on whatever evidence about that event pops into your mind, without taking into account the ways in which some pieces of evidence are more memorable than others, or some pieces of evidence are easier to come by than others. This bias directly consists in considering a mismatched data set that leads to a distorted model, and biased estimate.

Qualitatively Confused - Using qualitative, binary reasoning may make it easier to confuse belief and reality; if we use probability distributions, the distinction is much clearer.

That Alien Message - Einstein used evidence more efficiently than other physicists, but he was still extremely inefficient in an absolute sense. If a huge team of cryptographers and physicists were examining a interstellar transmission, going over it bit by bit, we could deduce principles on the order of Galilean gravity just from seeing one or two frames of a picture. As if the very first human to see an apple fall, had, on the instant, realized that its position went as the square of the time and that this implied constant acceleration.

Changing the Definition of Science - Many of these ideas are surprisingly conventional, and being floated around by other thinkers. I'm a good deal less of a lonely iconoclast than I seem; maybe it's just the way I talk.

Belief in Belief - Suppose someone claims to have a dragon in their garage, but as soon as you go to look, they say, "It's an invisible dragon!" The remarkable thing is that they know in advance exactly which experimental results they shall have to excuse, indicating that some part of their mind knows what's really going on. And yet they may honestly believe they believe there's a dragon in the garage. They may perhaps believe it is virtuous to believe there is a dragon in the garage, and believe themselves virtuous. Even though they anticipate as if there is no dragon.

Belief as Attire - When you've stopped anticipating-as-if something, but still believe it is virtuous to believe it, this does not create the true fire of the child who really does believe. On the other hand, it is very easy for people to be passionate about group identification - sports teams, political sports teams - and this may account for the passion of beliefs worn as team-identification attire.

Conjunction Fallacy - Elementary probability theory tells us that the probability of one thing (we write P(A)) is necessarily greater than or equal to the conjunction of that thing and another thing (write P(A&B)). However, in the psychology lab, subjects' judgments do not conform to this rule. This is not an isolated artifact of a particular study design. Debiasing won't be as simple as practicing specific questions, it requires certain general habits of thought.

Making Beliefs Pay Rent (in Anticipated Experiences) - Not every belief that we have is directly about sensory experience, but beliefs should pay rent in anticipations of experience. For example, if I believe that "Gravity is 9.8 m/s^2" then I should be able to predict where I'll see the second hand on my watch at the time I hear the crash of a bowling ball dropped off a building. On the other hand, if your postmodern English professor says that the famous writer Wulky is a "post-utopian", this may not actually mean anything. The moral is to ask "What experiences do I anticipate?" not "What statements do I believe?"

Decoherence - A quantum system that factorizes can evolve into a system that doesn't factorize, destroying the illusion of independence. But entangling a quantum system with its environment, can appear to destroy entanglements that are already present. Entanglement with the environment can separate out the pieces of an amplitude distribution, preventing them from interacting with each other. Decoherence is fundamentally symmetric in time, but appears asymmetric because of the second law of thermodynamics.

On Being Decoherent - When a sensor measures a particle whose amplitude distribution stretches over space - perhaps seeing if the particle is to the left or right of some dividing line - then the standard laws of quantum mechanics call for the sensor+particle system to evolve into a state of (particle left, sensor measures LEFT) + (particle right, sensor measures RIGHT). But when we humans look at the sensor, it only seems to say "LEFT" or "RIGHT", never a mixture like "LIGFT". This, of course, is because we ourselves are made of particles, and subject to the standard quantum laws that imply decoherence. Under standard quantum laws, the final state is (particle left, sensor measures LEFT, human sees "LEFT") + (particle right, sensor measures RIGHT, human sees "RIGHT").

Decoherence is Pointless - There is no exact point at which decoherence suddenly happens. All of quantum mechanics is continuous and differentiable, and decoherent processes are no exception to this.

Decoherent Essences - Decoherence is implicit within physics, not an extra law on top of it. You can choose representations that make decoherence harder to see, just like you can choose representations that make apples harder to see, but exactly the same physical process still goes on; the apple doesn't disappear and neither does decoherence. If you could make decoherence magically go away by choosing the right representation, we wouldn't need to shield quantum computers from the environment.

Decoherence is Falsifiable and Testable - (Note: Designed to be standalone readable.) An epistle to the physicists. To probability theorists, words like "simple", "falsifiable", and "testable" have exact mathematical meanings, which are there for very strong reasons. The (minority?) faction of physicists who say that many-worlds is "not falsifiable" or that it "violates Occam's Razor" or that it is "untestable", are committing the same kind of mathematical crime as non-physicists who invent their own theories of gravity that go as inverse-cube. This is one of the reasons why I, a non-physicist, dared to talk about physics - because I saw (some!) physicists using probability theory in a way that was simply wrong. Not just criticizable, but outright mathematically wrong: 2 + 2 = 3.

Living in Many Worlds - The many worlds of quantum mechanics are not some strange, alien universe into which you have been thrust. They are where you have always lived. Egan's Law: "It all adds up to normality." Then why care about quantum physics at all? Because there's still the question of what adds up to normality, and the answer to this question turns out to be, "Quantum physics." If you're thinking of building any strange philosophies around many-worlds, you probably shouldn't - that's not what it's for.

The Error of Crowds - Variance decomposition does not imply majoritarian-ish results; this is an artifact of minimizing *square* error, and drops out using square root error when bias is larger than variance; how and why to factor in evidence requires more assumptions, as per Aumann agreement.

The Majority Is Always Wrong - Often, anything worse than the majority opinion should get selected out...so the majority opinion is often strictly superior to no others.

That Alien Message - Einstein used evidence more efficiently than other physicists, but he was still extremely inefficient in an absolute sense. If a huge team of cryptographers and physicists were examining a interstellar transmission, going over it bit by bit, we could deduce principles on the order of Galilean gravity just from seeing one or two frames of a picture. As if the very first human to see an apple fall, had, on the instant, realized that its position went as the square of the time and that this implied constant acceleration.

An Alien God - Evolution is awesomely powerful, unbelievably stupid, incredibly slow, monomaniacally singleminded, irrevocably splintered in focus, blindly shortsighted, and itself a completely accidental process. If evolution were a god, it would not be Jehovah, but H. P. Lovecraft's Azathoth, the blind idiot God burbling chaotically at the center of everything.

Adaptation-Executers, not Fitness-Maximizers - A central principle of evolutionary biology in general, and evolutionary psychology in particular. If we regarded human taste buds as trying to maximize fitness, we might expect that, say, humans fed a diet too high in calories and too low in micronutrients, would begin to find lettuce delicious, and cheeseburgers distasteful. But it is better to regard taste buds as an executing adaptation - they are adapted to an ancestral environment in which calories, not micronutrients, were the limiting factor.

Fake Optimization Criteria - Why study evolution? For one thing - it lets us see an alien optimization process up close - lets us see the real consequence of optimizing strictly for an alien optimization criterion like inclusive genetic fitness. Humans, who try to persuade other humans to do things their way, think that this policy criterion ought to require predators to restrain their breeding to live in harmony with prey; the true result is something that humans find less aesthetic.

Fake Utility Functions - Describes the seeming fascination that many have with trying to compress morality down to a single principle. The sequence leading up to this post tries to explain the cognitive twists whereby people smuggle all of their complicated other preferences into their choice of exactly which acts they try to justify using their single principle; but if they were really following only that single principle, they would choose other acts to justify.

Making Beliefs Pay Rent (in Anticipated Experiences) - Not every belief that we have is directly about sensory experience, but beliefs should pay rent in anticipations of experience. For example, if I believe that "Gravity is 9.8 m/s^2" then I should be able to predict where I'll see the second hand on my watch at the time I hear the crash of a bowling ball dropped off a building. On the other hand, if your postmodern English professor says that the famous writer Wulky is a "post-utopian", this may not actually mean anything. The moral is to ask "What experiences do I anticipate?" not "What statements do I believe?"

Belief in Belief - Suppose someone claims to have a dragon in their garage, but as soon as you go to look, they say, "It's an invisible dragon!" The remarkable thing is that they know in advance exactly which experimental results they shall have to excuse, indicating that some part of their mind knows what's really going on. And yet they may honestly believe they believe there's a dragon in the garage. They may perhaps believe it is virtuous to believe there is a dragon in the garage, and believe themselves virtuous. Even though they anticipate as if there is no dragon.

Timeless Causality - Using the modern, Bayesian formulation of causality, we can define causality without talking about time - define it purely in terms of relations. The river of time never flows, but it has a direction.

Thou Art Physics - If the laws of physics control everything we do, then how can our choices be meaningful? Because you are physics. You aren't competing with physics for control of the universe, you are within physics. Anything you control is necessarily controlled by physics.

Timeless Control - We throw away "time" but retain causality, and with it, the concepts "control" and "decide". To talk of something as having been "always determined" is mixing up a timeless and a timeful conclusion, with paradoxical results. When you take a perspective outside time, you have to be careful not to

let your old, timeful intuitions run wild in the absence of their
subject matter.

Futuristic Predictions as Consumable Goods - The Friedman Unit is named after Thomas Friedman who 8 times (between 2003 and 2007) called "the next six months" the critical period in Iraq. This is because future predictions are created and consumed in the now; they are used to create feelings of delicious goodness or delicious horror now, not provide useful future advice.

The Wonder of Evolution - ...is not how amazingly well it works, but that it works at all without a mind, brain, or the ability to think abstractly - that an entirely accidental process can produce complex designs. If you talk about how amazingly well evolution works, you're missing the point.

The Logical Fallacy of Generalization from Fictional Evidence - The Logical Fallacy of Generalization from Fictional Evidence consists in drawing the real-world conclusions based on statements invented and selected for the purpose of writing fiction. The data set is not at all representative of the real world, and in particular of whatever real-world phenomenon you need to understand to answer your real-world question. Considering this data set leads to an inadequate model, and inadequate answers.

Evolving to Extinction - Contrary to a naive view that evolution works for the good of a species, evolution says that genes which outreproduce their alternative alleles increase in frequency within a gene pool. It is entirely possible for genes which "harm" the species to outcompete their alternatives in this way - indeed, it is entirely possible for a species to evolve to extinction.

The Tragedy of Group Selectionism - A tale of how some pre-1960s biologists were led astray by expecting evolution to do smart, nice things like they would do themselves.

Conjuring An Evolution To Serve You - If you take the hens who lay the most eggs in each generation, and breed from them, you should get hens who lay more and more eggs. Sounds logical, right? But this selection may actually favor the most dominant hen, that pecked its way to the top of the pecking order at the expense of other hens. Such breeding programs produce hens that must be housed in individual cages, or they will peck each other to death. Jeff Skilling of Enron fancied himself an evolution-conjurer - summoning the awesome power of evolution to work for him - and so, every year, every Enron employee's performance would be evaluated, and the bottom 10% would get fired, and the top performers would get huge raises and bonuses...

Anthropomorphic Optimism - You shouldn't bother coming up with clever, persuasive arguments for why evolution will do things the way you prefer. It really isn't listening.

Policy Debates Should Not Appear One-Sided - Debates over outcomes with multiple effects will have arguments both for and against, so you must integrate the evidence, not expect the issue to be completely one-sided.

The Scales of Justice, the Notebook of Rationality - People have an irrational tendency to simplify their assessment of things into how good or bad they are without considering that the things in question may have many distinct and unrelated attributes.

The Affect Heuristic - Positive and negative emotional impressions exert a greater effect on many decisions than does rational analysis.

The Halo Effect - Positive qualities seem to correlate with each other, whether or not they actually do.

Your Strength as a Rationalist - A hypothesis that forbids nothing, permits everything, and thereby fails to constrain anticipation. Your strength as a rationalist is your ability to be more confused by fiction than by reality. If you are equally good at explaining any outcome, you have zero knowledge.

Hindsight bias - Describes the tendency to seem much more likely in hindsight than could have been predicted beforehand.

Fake Optimization Criteria - Why study evolution? For one thing - it lets us see an alien optimization process up close - lets us see the real consequence of optimizing strictly for an alien optimization criterion like inclusive genetic fitness. Humans, who try to persuade other humans to do things their way, think that this policy criterion ought to require predators to restrain their breeding to live in harmony with prey; the true result is something that humans find less aesthetic.

Belief in Belief - Suppose someone claims to have a dragon in their garage, but as soon as you go to look, they say, "It's an invisible dragon!" The remarkable thing is that they know in advance exactly which experimental results they shall have to excuse, indicating that some part of their mind knows what's really going on. And yet they may honestly believe they believe there's a dragon in the garage. They may perhaps believe it is virtuous to believe there is a dragon in the garage, and believe themselves virtuous. Even though they anticipate as if there is no dragon.

Self-deception: Hypocrisy or Akrasia? - It is suggested that in some cases, people who say one thing and do another thing are not in fact "hypocrites". Instead they are suffering from "akrasia" or weakness of will. At the end, the problem of deciding what parts of a person's mind are considered their "real self" is discussed.

Belief as Attire - When you've stopped anticipating-as-if something, but still believe it is virtuous to believe it, this does not create the true fire of the child who really does believe. On the other hand, it is very easy for people to be passionate about group identification - sports teams, political sports teams - and this may account for the passion of beliefs worn as team-identification attire.

Joy in the Merely Real - If you can't take joy in things that turn out to be explicable, you're going to set yourself up for eternal disappointment. Don't worry if quantum physics turns out to be normal.

Joy in the Merely Real - If you can't take joy in things that turn out to be explicable, you're going to set yourself up for eternal disappointment. Don't worry if quantum physics turns out to be normal.

No Safe Defense, Not Even Science - Why am I trying to break your trust in Science? Because you can't think and trust at the same time. The social rules of Science are verbal rather than quantitative; it is possible to believe you are following them. With Bayesianism, it is never possible to do an exact calculation and get the exact rational answer that you know exists. You are visibly less than perfect, and so you will not be tempted to trust yourself.

Planning Fallacy - We tend to plan envisioning that everything will go as expected. Even assuming that such an estimate is accurate conditional on everything going as expected, things will not go as expected. As a result, we routinely see outcomes worse then the ex ante worst case scenario.

Planning Fallacy - We tend to plan envisioning that everything will go as expected. Even assuming that such an estimate is accurate conditional on everything going as expected, things will not go as expected. As a result, we routinely see outcomes worse then the ex ante worst case scenario.

Policy Debates Should Not Appear One-Sided - Debates over outcomes with multiple effects will have arguments both for and against, so you must integrate the evidence, not expect the issue to be completely one-sided.

Policy Debates Should Not Appear One-Sided - Debates over outcomes with multiple effects will have arguments both for and against, so you must integrate the evidence, not expect the issue to be completely one-sided.

The Scales of Justice, the Notebook of Rationality - People have an irrational tendency to simplify their assessment of things into how good or bad they are without considering that the things in question may have many distinct and unrelated attributes.

Correspondence Bias - Also known as the fundamental attribution error, refers to the tendency to attribute the behavior of others to intrinsic dispositions, while excusing one's own behavior as the result of circumstance.

Useless Medical Disclaimers - Medical disclaimers without probabilities are hard to use, and if probabilities aren't there because some people can't handle having there, maybe we ought to tax those people.

Futuristic Predictions as Consumable Goods - The Friedman Unit is named after Thomas Friedman who 8 times (between 2003 and 2007) called "the next six months" the critical period in Iraq. This is because future predictions are created and consumed in the now; they are used to create feelings of delicious goodness or delicious horror now, not provide useful future advice.

Science Doesn't Trust Your Rationality - The reason Science doesn't always agree with the exact, Bayesian, rational answer, is that Science doesn't trust you to be rational. It wants you to go out and gather overwhelming experimental evidence.

Think Like Reality - "Quantum physics is not "weird". You are weird. You have the absolutely bizarre idea that reality ought to consist of little billiard balls bopping around, when in fact reality is a perfectly normal cloud of complex amplitude in configuration space. This is your problem, not reality's, and you are the one who needs to change."

Conjunction Fallacy - Elementary probability theory tells us that the probability of one thing (we write P(A)) is necessarily greater than or equal to the conjunction of that thing and another thing (write P(A&B)). However, in the psychology lab, subjects' judgments do not conform to this rule. This is not an isolated artifact of a particular study design. Debiasing won't be as simple as practicing specific questions, it requires certain general habits of thought.

Science Doesn't Trust Your Rationality - The reason Science doesn't always agree with the exact, Bayesian, rational answer, is that Science doesn't trust you to be rational. It wants you to go out and gather overwhelming experimental evidence.

No Safe Defense, Not Even Science - Why am I trying to break your trust in Science? Because you can't think and trust at the same time. The social rules of Science are verbal rather than quantitative; it is possible to believe you are following them. With Bayesianism, it is never possible to do an exact calculation and get the exact rational answer that you know exists. You are visibly less than perfect, and so you will not be tempted to trust yourself.

We Change Our Minds Less Often Than We Think - We all change our minds occasionally, but we don't constantly, honestly reevaluate every decision and course of action. Once you think you believe something, the chances are good that you already do, for better or worse.

Hold Off On Proposing Solutions - Proposing Solutions Prematurely is dangerous, because it introduces weak conclusions in the pool of the facts you are considering, and as a result the data set you think about becomes weaker, overly tilted towards premature conclusions that are likely to be wrong, that are less representative of the phenomenon you are trying to model than the initial facts you started from, before coming up with the premature conclusions.

Asch's Conformity Experiment - The unanimous agreement of surrounding others can make subjects disbelieve (or at least, fail to report) what's right before their eyes. The addition of just one dissenter is enough to dramatically reduce the rates of improper conformity.

Self-deception: Hypocrisy or Akrasia? - It is suggested that in some cases, people who say one thing and do another thing are not in fact "hypocrites". Instead they are suffering from "akrasia" or weakness of will. At the end, the problem of deciding what parts of a person's mind are considered their "real self" is discussed.

The Dilemma: Science or Bayes? - The failure of first-half-of-20th-century-physics was not due to straying from the scientific method. Science and rationality - that is, Science and Bayesianism - aren't the same thing, and sometimes they give different answers.

Science Doesn't Trust Your Rationality - The reason Science doesn't always agree with the exact, Bayesian, rational answer, is that Science doesn't trust you to be rational. It wants you to go out and gather overwhelming experimental evidence.

The Wonder of Evolution - ...is not how amazingly well it works, but that it works at all without a mind, brain, or the ability to think abstractly - that an entirely accidental process can produce complex designs. If you talk about how amazingly well evolution works, you're missing the point.

Adaptation-Executers, not Fitness-Maximizers - A central principle of evolutionary biology in general, and evolutionary psychology in particular. If we regarded human taste buds as trying to maximize fitness, we might expect that, say, humans fed a diet too high in calories and too low in micronutrients, would begin to find lettuce delicious, and cheeseburgers distasteful. But it is better to regard taste buds as an executing adaptation - they are adapted to an ancestral environment in which calories, not micronutrients, were the limiting factor.

Think Like Reality - "Quantum physics is not "weird". You are weird. You have the absolutely bizarre idea that reality ought to consist of little billiard balls bopping around, when in fact reality is a perfectly normal cloud of complex amplitude in configuration space. This is your problem, not reality's, and you are the one who needs to change."

Your Strength as a Rationalist - A hypothesis that forbids nothing, permits everything, and thereby fails to constrain anticipation. Your strength as a rationalist is your ability to be more confused by fiction than by reality. If you are equally good at explaining any outcome, you have zero knowledge.

Making Beliefs Pay Rent (in Anticipated Experiences) - Not every belief that we have is directly about sensory experience, but beliefs should pay rent in anticipations of experience. For example, if I believe that "Gravity is 9.8 m/s^2" then I should be able to predict where I'll see the second hand on my watch at the time I hear the crash of a bowling ball dropped off a building. On the other hand, if your postmodern English professor says that the famous writer Wulky is a "post-utopian", this may not actually mean anything. The moral is to ask "What experiences do I anticipate?" not "What statements do I believe?"

Reductionism - We build models of the universe that have many different levels of description. But so far as anyone has been able to determine, the universe itself has only the single level of fundamental physics - reality doesn't explicitly compute protons, only quarks.

Configurations and Amplitude - A preliminary glimpse at the stuff reality is made of. The classic split-photon experiment with half-silvered mirrors. Alternative pathways the photon can take, can cancel each other out. The mysterious measuring tool that tells us the relative squared moduli.

Joint Configurations - The laws of physics are inherently over mathematical entities, configurations, that involve multiple particles. A basic, ontologically existent entity, according to our current understanding of quantum mechanics, does not look like a photon - it looks like a configuration of the universe with "A photon here, a photon there." Amplitude flows between these configurations can cancel or add; this gives us a way to detect which configurations are distinct. It is an experimentally testable fact that "Photon 1 here, photon 2 there" is the same configuration as "Photon 2 here, photon 1 there".

Distinct Configurations - Since configurations are over the combined state of all the elements in a system, adding a sensor that detects whether a particle went one way or the other, becomes a new element of the system that can make configurations "distinct" instead of "identical". This confused the living daylights out of early quantum experimenters, because it meant that things behaved differently when they tried to "measure" them. But it's not only measuring instruments that do the trick - any sensitive physical element will do - and the distinctness of configurations is a physical fact, not a fact about our knowledge. There is no need to suppose that the universe cares what we think.

Where Philosophy Meets Science - In retrospect, supposing that quantum physics had anything to do with consciousness was a big mistake. Could philosophers have told the physicists so? But we don't usually see philosophers sponsoring major advances in physics; why not?

Can You Prove Two Particles Are Identical? - You wouldn't think that it would be possible to do an experiment that told you that two particles are completely identical - not just to the limit of experimental precision, but perfectly. You could even give a precise-sounding philosophical argument for why it was not possible - but the argument would have a deeply buried assumption. Quantum physics violates this deep assumption, making the experiment easy.

Classical Configuration Spaces - How to visualize the state of a system of two 1-dimensional particles, as a single point in 2-dimensional space. A preliminary step before moving into...

The Quantum Arena - Instead of a system state being associated with a single point in a classical configuration space, the instantaneous real state of a quantum system is a complex amplitude distribution over a quantum configuration space. What creates the illusion of "individual particles", like an electron caught in a trap, is a plaid distribution - one that happens to factor into the product of two parts. It is the whole distribution that evolves when a quantum system evolves. Individual configurations don't have physics; amplitude distributions have physics. Quantum entanglement is the general case; quantum independence is the special case.

Feynman Paths - Instead of thinking that a photon takes a single straight path through space, we can regard it as taking all possible paths through space, and adding the amplitudes for every possible path. Nearly all the paths cancel out - unless we do clever quantum things, so that some paths add instead of canceling out. Then we can make light do funny tricks for us, like reflecting off a mirror in such a way that the angle of incidence doesn't equal the angle of reflection. But ordinarily, nearly all the paths except an extremely narrow band, cancel out - this is one of the keys to recovering the hallucination of classical physics.

No Individual Particles - One of the chief ways to confuse yourself while thinking about quantum mechanics, is to think as if photons were little billiard balls bouncing around. The appearance of little billiard balls is a special case of a deeper level on which there are only multiparticle configurations and amplitude flows. It is easy to set up physical situations in which there exists no fact of the matter as to which electron was originally which.

Decoherence - A quantum system that factorizes can evolve into a system that doesn't factorize, destroying the illusion of independence. But entangling a quantum system with its environment, can appear to destroy entanglements that are already present. Entanglement with the environment can separate out the pieces of an amplitude distribution, preventing them from interacting with each other. Decoherence is fundamentally symmetric in time, but appears asymmetric because of the second law of thermodynamics.

Which Basis Is More Fundamental? - The position basis can be computed locally in the configuration space; the momentum basis is not local. Why care about locality? Because it is a very deep principle; reality itself seems to favor it in some way.

Where Physics Meets Experience - Meet the Ebborians, who reproduce by fission. The Ebborian brain is like a thick sheet of paper that splits down its thickness. They frequently experience dividing into two minds, and can talk to their other selves. It seems that their unified theory of physics is almost finished, and can answer every question, when one Ebborian asks: When exactly does one Ebborian become two people?

Where Experience Confuses Physicists - It then turns out that the entire planet of Ebbore is splitting along a fourth-dimensional thickness, duplicating all the people within it. But why does the apparent chance of "ending up" in one of those worlds, equal the square of the fourth-dimensional thickness? Many mysterious answers are proposed to this question, and one non-mysterious one.

On Being Decoherent - When a sensor measures a particle whose amplitude distribution stretches over space - perhaps seeing if the particle is to the left or right of some dividing line - then the standard laws of quantum mechanics call for the sensor+particle system to evolve into a state of (particle left, sensor measures LEFT) + (particle right, sensor measures RIGHT). But when we humans look at the sensor, it only seems to say "LEFT" or "RIGHT", never a mixture like "LIGFT". This, of course, is because we ourselves are made of particles, and subject to the standard quantum laws that imply decoherence. Under standard quantum laws, the final state is (particle left, sensor measures LEFT, human sees "LEFT") + (particle right, sensor measures RIGHT, human sees "RIGHT").

The Conscious Sorites Paradox - Decoherence is implicit in quantum physics, not an extra law on top of it. Asking exactly when "one world" splits into "two worlds" may be like asking when, if you keep removing grains of sand from a pile, it stops being a "heap". Even if you're inside the world, there may not be a definite answer. This puzzle does not arise only in quantum physics; the Ebborians could face it in a classical universe, or we could build sentient flat computers that split down their thickness. Is this really a physicist's problem?

Decoherence is Pointless - There is no exact point at which decoherence suddenly happens. All of quantum mechanics is continuous and differentiable, and decoherent processes are no exception to this.

Decoherent Essences - Decoherence is implicit within physics, not an extra law on top of it. You can choose representations that make decoherence harder to see, just like you can choose representations that make apples harder to see, but exactly the same physical process still goes on; the apple doesn't disappear and neither does decoherence. If you could make decoherence magically go away by choosing the right representation, we wouldn't need to shield quantum computers from the environment.

The Born Probabilities - The last serious mysterious question left in quantum physics: When a quantum world splits in two, why do we seem to have a greater probability of ending up in the larger blob, exactly proportional to the integral of the squared modulus? It's an open problem, but non-mysterious answers have been proposed. Try not to go funny in the head about it.

Decoherence as Projection - Since quantum evolution is linear and unitary, decoherence can be seen as projecting a wavefunction onto orthogonal subspaces. This can be neatly illustrated using polarized photons and the angle of the polarized sheet that will absorb or transmit them.

Entangled Photons - Using our newly acquired understanding of photon polarizations, we see how to construct a quantum state of two photons in which, when you measure one of them, the person in the same world as you, will always find that the opposite photon has opposite quantum state. This is not because any influence is transmitted; it is just decoherence that takes place in a very symmetrical way, as can readily be observed in our calculations.

Bell's Theorem: No EPR "Reality" - (Note: This post was designed to be read as a stand-alone, if desired.) Originally, the discoverers of quantum physics thought they had discovered an incomplete description of reality - that there was some deeper physical process they were missing, and this was why they couldn't predict exactly the results of quantum experiments. The math of Bell's Theorem is surprisingly simple, and we walk through it. Bell's Theorem rules out being able to locally predict a single, unique outcome of measurements - ruling out a way that Einstein, Podolsky, and Rosen once defined "reality". This shows how deep implicit philosophical assumptions can go. If worlds can split, so that there is no single unique outcome, then Bell's Theorem is no problem. Bell's Theorem does, however, rule out the idea that quantum physics describes our partial knowledge of a deeper physical state that could locally produce single outcomes - any such description will be inconsistent.

Spooky Action at a Distance: The No-Communication Theorem - As Einstein argued long ago, the quantum physics of his era - that is, the single-global-world interpretation of quantum physics, in which experiments have single unique random results - violates Special Relativity; it imposes a preferred space of simultaneity and requires a mysterious influence to be transmitted faster than light; which mysterious influence can never be used to transmit any useful information. Getting rid of the single global world dispels this mystery and puts everything back to normal again.

Quantum Non-Realism - "Shut up and calculate" is the best approach you can take when none of your theories are very good. But that is not the same as claiming that "Shut up!" actually is a theory of physics. Saying "I don't know what these equations mean, but they seem to work" is a very different matter from saying: "These equations definitely don't mean anything, they just work!"

Collapse Postulates - Early physicists simply didn't think of the possibility of more than one world - it just didn't occur to them, even though it's the straightforward result of applying the quantum laws at all levels. So they accidentally invented a completely and strictly unnecessary part of quantum theory to ensure there was only one world - a law of physics that says that parts of the wavefunction mysteriously and spontaneously disappear when decoherence prevents us from seeing them any more. If such a law really existed, it would be the only non-linear, non-unitary, non-differentiable, non-local, non-CPT-symmetric, acausal, faster-than-light phenomenon in all of physics.

If Many-Worlds Had Come First - If early physicists had never made the mistake, and thought immediately to apply the quantum laws at all levels to produce macroscopic decoherence, then "collapse postulates" would today seem like a completely crackpot theory. In addition to their other problems, like FTL, the collapse postulate would be the only physical law that was informally specified - often in dualistic (mentalistic) terms - because it was the only fundamental law adopted without precise evidence to nail it down. Here, we get a glimpse at that alternate Earth.

Many Worlds, One Best Guess - Summarizes the arguments that nail down macroscopic decoherence, aka the "many-worlds interpretation". Concludes that many-worlds wins outright given the current state of evidence. The argument should have been over fifty years ago. New physical evidence could reopen it, but we have no particular reason to expect this.

Living in Many Worlds - The many worlds of quantum mechanics are not some strange, alien universe into which you have been thrust. They are where you have always lived. Egan's Law: "It all adds up to normality." Then why care about quantum physics at all? Because there's still the question of what adds up to normality, and the answer to this question turns out to be, "Quantum physics." If you're thinking of building any strange philosophies around many-worlds, you probably shouldn't - that's not what it's for.

Mach's Principle: Anti-Epiphenomenal Physics - Could you tell if the whole universe were shifted an inch to the left? Could you tell if the whole universe was traveling left at ten miles per hour? Could you tell if the whole universe was accelerating left at ten miles per hour? Could you tell if the whole universe was rotating?

Timeless Physics - What time is it? How do you know? The question "What time is it right now?" may make around as much sense as asking "Where is the universe?" Not only that, our physics equations may not need a t in them!

Timeless Beauty - To get rid of time you must reduce it to nontime. In timeless physics, everything that exists is perfectly global or perfectly local. The laws of physics are perfectly global; the configuration space is perfectly local. Every fundamentally existent ontological entity has a unique identity and a unique value. This beauty makes ugly theories much more visibly ugly; a collapse postulate becomes a visible scar on the perfection.

Timeless Causality - Using the modern, Bayesian formulation of causality, we can define causality without talking about time - define it purely in terms of relations. The river of time never flows, but it has a direction.

Timeless Identity - How can you be the same person tomorrow as today, in the river that never flows, when not a drop of water is shared between one time and another? Having used physics to completely trash all naive theories of identity, we reassemble a conception of persons and experiences from what is left. With a surprising practical application...

Thou Art Physics - If the laws of physics control everything we do, then how can our choices be meaningful? Because you are physics. You aren't competing with physics for control of the universe, you are within physics. Anything you control is necessarily controlled by physics.

Timeless Control - We throw away "time" but retain causality, and with it, the concepts "control" and "decide". To talk of something as having been "always determined" is mixing up a timeless and a timeful conclusion, with paradoxical results. When you take a perspective outside time, you have to be careful not to

let your old, timeful intuitions run wild in the absence of their
subject matter.

A short story set in the same world as <a href="/lw/p1/initiation_ceremony/">Initiation Ceremony</a>. Future physics students look back on the cautionary tale of quantum physics.

The Dilemma: Science or Bayes? - The failure of first-half-of-20th-century-physics was not due to straying from the scientific method. Science and rationality - that is, Science and Bayesianism - aren't the same thing, and sometimes they give different answers.

Science Doesn't Trust Your Rationality - The reason Science doesn't always agree with the exact, Bayesian, rational answer, is that Science doesn't trust you to be rational. It wants you to go out and gather overwhelming experimental evidence.

When Science Can't Help - If you have an idea, Science tells you to test it experimentally. If you spend 10 years testing the idea and the result comes out negative, Science slaps you on the back and says, "Better luck next time." If you want to spend 10 years testing a hypothesis that will actually turn out to be right, you'll have to try to do the thing that Science doesn't trust you to do: think rationally, and figure out the answer before you get clubbed over the head with it.

Science Isn't Strict Enough - Science lets you believe any damn stupid idea that hasn't been refuted by experiment. Bayesianism says there is always an exactly rational degree of belief given your current evidence, and this does not shift a nanometer to the left or to the right depending on your whims. Science is a social freedom - we let people test whatever hypotheses they like, because we don't trust the village elders to decide in advance - but you shouldn't confuse that with an individual standard of rationality.

No Safe Defense, Not Even Science - Why am I trying to break your trust in Science? Because you can't think and trust at the same time. The social rules of Science are verbal rather than quantitative; it is possible to believe you are following them. With Bayesianism, it is never possible to do an exact calculation and get the exact rational answer that you know exists. You are visibly less than perfect, and so you will not be tempted to trust yourself.

Changing the Definition of Science - Many of these ideas are surprisingly conventional, and being floated around by other thinkers. I'm a good deal less of a lonely iconoclast than I seem; maybe it's just the way I talk.

Faster Than Science - Is it really possible to arrive at the truth faster than Science does? Not only is it possible, but the social process of science relies on scientists doing so - when they choose which hypotheses to test. In many answer spaces it's not possible to find the true hypothesis by accident. Science leaves it up to experiment to socially declare who was right, but if there weren't some people who could get it right in the absence of overwhelming experimental proof, science would be stuck.

Einstein's Speed - Albert was unusually good at finding the right theory in the presence of only a small amount of experimental evidence. Even more unusually, he admitted it - he claimed to know the theory was right, even in advance of the public proof. It's possible to arrive at the truth by thinking great high-minded thoughts of the sort that Science does not trust you to think, but it's a lot harder than arriving at the truth in the presence of overwhelming evidence.

That Alien Message - Einstein used evidence more efficiently than other physicists, but he was still extremely inefficient in an absolute sense. If a huge team of cryptographers and physicists were examining a interstellar transmission, going over it bit by bit, we could deduce principles on the order of Galilean gravity just from seeing one or two frames of a picture. As if the very first human to see an apple fall, had, on the instant, realized that its position went as the square of the time and that this implied constant acceleration.

Einstein's Superpowers - There's an unfortunate tendency to talk as if Einstein had superpowers - as if, even before Einstein was famous, he had an inherent disposition to be Einstein - a potential as rare as his fame and as magical as his deeds. Yet the way you acquire superpowers is not by being born with them, but by seeing, with a sudden shock, that they are perfectly normal.

Class Project - From the world of Initiation Ceremony. Brennan and the others are faced with their midterm exams.

Why Quantum? - Why do a series on quantum mechanics? Some of the many morals that are best illustrated by the tale of quantum mechanics and its misinterpretation.</div>